Nuclear fuel assembly that has been combusted in a nuclear reactor for a prescribed duration, that is, the so-called used nuclear fuel assembly, is cooled for a predetermined period of time in a cooling pit of an atomic power plant. Further, the used nuclear fuel assembly is accommodated in a cask, which is a container for transportation, and transported to a storage and recycling facility, where they are stored. To accommodate used nuclear fuel assembly in the cask, there is employed a holding container having a lattice-like section (called “basket”), which has a plurality of accommodation chambers as cells for the used nuclear fuel assemblies to be inserted therein one by one, with ensured adequate holding forces such as against vibrations during transportation.
In the conventional basket, as shown in FIG. 16, longitudinal and transverse plate-like members 1 are alternately combined by engagement between slits 2 formed therein, to provide a lattice-like section for used nuclear fuel assemblies to be inserted therein. In an employed plate-like member 1, as a base material la there is an aluminum alloy 10 mm or near in thickness and having an excellent characteristic in strength, such as in Al—Cu alloys specified by JIS2219 or Al—Mg alloys specified by JIS5083, for example, and on a surface thereof is affixed a plate member (a nuclear absorbing material) 1 mm or near in thickness and made of Al—B alloy having a neutron absorption ability.
Such an affix structure is employed because the neutron absorbing material is low of workability and difficult to be solely used as a structural member. In general, the plate-like members 1 have a width ranging 300 to 350 mm or near.
However, the plate-like member 1 used in the conventional basket in which a neutron absorbing material 3 is affixed on the aluminum alloy base material la requires much time for manufacture and also the material is costly. By the way, affixation of the neutron absorbing material 3 to the base material is performed by spot welding, screw fastening, or riveting. Further, in general, a few thousands of plate-like members 1 are necessary for manufacture of baskets to be accommodated in a single cask.
Further, in the conventional plate-like member 1, there can develop a step between the base material la and the neutron absorbing material 3 affixed thereon. It is know from experience that, the used nuclear fuel assembly gets caught create problem during their insertion or removal. Moreover, in the case of affix by a spot welding, deterioration in a long-term use may cause the neutron absorbing material 3 to exfoliate, as another problem. Accordingly, it is desirable to solely use Al—B alloy having a neutron absorption ability to make the baskets.
Conventionally, dissolution methods are used for manufacture of an Al—B alloy. However, the liquid phase line temperature rises steeply as the quantity of added B (Boron) (hereafter, addition quantity of B) increases. Therefore, B is added as powder or in the form of Al—B alloy into the Al (Aluminum) alloy, or added in the form of a boron compound such as KBF4 into molten Al to produce an Al—B inter-metal compound, or those by a casting from a solid-liquid coexisting region under the liquid phase line temperature, or by way of a casting under pressure, with various improvements for enhanced mechanical properties such as strength and ductility.
There are many such improvements, for example, Japanese Patent Application Laid-Open Publication Nos. 59-501672, 61-235523, 62-70799, 62-235437, 62-243733, 63-312943, 1-312043, 1-312044, 9-165637, etc.
In Al—B alloys manufactured using the dissolution methods, upon addition of B that absorbs neutrons, if there exist inter-metal compounds of AlB2 and AlB12 as B compounds, in particular if there exist much AlB12, then the workability is reduced. However, it is difficult to control the quantity of AlB12 from the currently available technology. As a consequence, 1.5 weight % is the limit as a quantity of B to be added as a practical material. However, with this amount, there is a drawback that the effect of neutron absorption is small.
Instead of Al—B alloys “Boral” may be used as the material for neutron absorption. Boral is a sandwiched and pressed material of powder having 30–40 weight percentage of B4C mixed in Al base material. However, the tensile strength of Boral is about 40 Mpa and thus it is very low, extension is about 1% and thus small, and further it is difficult to mold. As a consequence, the reality is that, Boral has not been used as structural material till present.
As another manufacturing method of Al—B4C composite material, there is use of a power sintering method, in which Al alloy and B4C, both as powder, are uniformly mixed and solidified for formation, and which can avoid problems described in conjunction with dissolution, in addition to having merits such as the possibility of more flexible selection of matrix compound.
In U.S. Pat. No. 5,486,223 and a series of subsequent inventions by the same inventors, there are described methods of using a powder metallurgical method to obtain an Al—B4C composite material excellent in strength characteristic. In particular, U.S. Pat. No. 5,700,962 mainly addresses manufacture of a neutron shielding material.
In those inventions, however, there is employed a special B4C having a particular element added to enhance the binding with matrix, and the process also is complex, as problems significant in cost for practice in industrial scale. Further, there are anxieties in performance such that a porous formed body of powder simply hardened by CIP is heated and extruded, accompanying gas intrusion, and that some matrix composition is exposed to high temperatures over 625° C., when sintering a billet, with resultant significant deterioration of characteristic.
As described, Al alloys manufactured by dissolution method had a limit in quantity of addition of a compound having neutron absorption power, such as B, and the neutron absorption effect was small. For solution thereto, the above-noted many inventions were made, with prerequisites for practice, such as dissolution of a base alloy having controlled proportions to the extent of contained compound phases (AlB2, AlB12, etc.) as well, and use of a very expensive condensed boron, causing a great increase in production cost, with a difficulty of practice in industrial scale.
In regard of operation also, there were problems such as contamination in reactor (with the need of a reactor cleaning to remove dross of high B concentration, as contamination by stagnation such as of fluorides thrown in, etc.), and damages to reactor materials due to a high dissolution temperature (needing sometime 1200° C. or more), practically with the impossibility of execution in ordinary Al oriented dissolution facilities.
As to the Boral of which the B4C content is as high as 30–40 weight percentage, because of the problem of workability, the use as a structural material is impossible.
On such background, it has been desirable to implement an aluminum composite material that, by in crease in B content, has a high neutron absorption ability, as a matter of course, and excellent mechanical properties such as tensile strength and extension, and is easy of machining, to be applicable as a structural material with a neutron absorption ability, as well as a manufacturing method therefor.